The emission of oxides of nitrogen (NO.sub.x) from the exhaust of internal combustion engines is a direct function of the combustion temperatures of such engines and does not become particularly objectionable until such combustion temperatures exceed about 2400.degree. F. However, such temperatures frequently increase to about 3500.degree. F. under certain operating conditions. It is known that by recirculating a certain percentage of the exhaust gas back through the engine, enough dilution can be achieved to reduce the combustion temperatures and thus decrease NO.sub.x emissions to an acceptable level.
Numerous systems have been developed for recycling the exhaust gas through spark ignition engines. However, such systems utilize the vacuum created by the restriction of the intake air necessary to obtain the proper air-to-fuel mixture in such spark ignition engines.
Compression ignition or diesel engines, on the other hand, do not restrict intake air, and thus do not have the required vacuum necessary to regulate exhaust gas recirculation. Consequently, the apparatus typically used on spark ignition engines are not particularly applicable for such diesel engines.